Who or what is "Black Kingdom"

I've seen that phrase printed on some of the guns, like my Snowstorm Tesla rifle, as well as the recent Scylla and Charybdis Tesla rifles. I thought that it might be an ad for a related game, or perhaps a favorite band, maybe even the name of a digital art studio, but a Google search didn't return anything that seemed strong enough to say, "Ah, that's it."